A Franck-Hertz tube is filled with lithium vapour instead of mercury
PHYSICS · Multi-part · Fermi University Physics · Quantum Mechanics · Origins of quantum theory · Origins of quantum theory
Quantum Mechanics: From the Photon to the Hydrogen Atom, Ch Line spectra, the Bohr atom and the old quantum theory
Problem
A Franck-Hertz tube is filled with lithium vapour instead of mercury. Electrons leave the cathode with negligible kinetic energy, are accelerated through a potential difference V, and must cross a small retarding voltage to reach the collector. Elastic collisions with the atoms transfer negligible energy (the electron is far lighter than a lithium atom), and the only atomic level within reach is the first excited level, which lies 1.85 eV above the ground state. An excited atom returns to the ground state by emitting a single photon. Take hc=1240 eV nm.
((a)) Predict the spacing, in volts, between successive dips of the collector current as V is raised.
((b)) The accelerating voltage can be raised only as far as 10.0 V before the supply reaches its limit. How many dips would be seen by then?
((c)) Predict the wavelength of the light the vapour emits once V has passed the first dip, and say whether it falls inside the visible range (380 to 750 nm).